1. Field of the Invention
The invention relates to a housing for an implantable tissue stimulating device, especially an implantable heart stimulation device.
2. Background of the Invention and Related Art
Heart stimulation devices for human beings, such as pacemakers are usually implanted in the chest, generally on the left side, a short distance below the clavicle or collar bone. The device will then rest between the pectoralis major and the skin. An electrode lead supplies electric pulses to the heart from the device. The electrode lead includes one or more electrodes, a connector at the proximal end of the electrode lead for connection of the lead to the stimulation device conductor(s) between the connector pin and the electrode(s), one of which typically is located at the distal end of the lead and insulation. The electrode lead is typically passed upwardly over the clavicle and is connected to the heart through a vein adjacent to the clavicle. Usually, the housing of the heart stimulating device is made of conductive material, and electric pulses to the heart are delivered by means of a distal electrode of the electrode lead, being located in the heart in electrical contact with the tissue. The housing is implanted so as to be in electrical contact with the surrounding tissue, and constitutes a second electrode surface. The object is to create an electrical field at the interface of the distal electrode of the electrode lead (stimulation electrode) with the underlying myocardium. However, in a device as described, an electrical field will also be created between the stimulation electrode and the housing. This may, depending on the strength of the field, cause unwanted stimulation of other muscles subjected to the field, e.g. the pectoralis major. To avoid this effect, parts of the housing can be provided with an insulating cover, e.g. of parylene. Usually the whole housing except for a contact window in a central portion of the side of the housing facing the skin, when implanted, is covered with parylene. In such a case the electric field from the housing will be concentrated to the area of the window.
U.S. Pat. No. 5,480,416 refers to such devices as prior art, and further discloses a pacemaker having two sides covered with parylene, while an edge joining the two sides is uninsulated, and constitutes a contact surface. Thus this pacemaker can be implanted with either of its two sides facing the skin of its carrier.
U.S. Pat. No. 5,658,321 discloses an implantable defibrillator or pulse generator having an electrically conductive housing used as an electrode. The exterior surface of the housing is provided with ridges. Hereby the surface area is increased, and the electrical resistance in the housing/tissue interface is decreased. By the arrangement of sharp corners in connection with the ridges, a further resistance reduction is achieved.
A further casing for a power supply and pulsation control circuitry of a cardiac pacer is disclosed in U.S. Pat. No. 4,094,321. This casing has a substantially flat bottom surface and a shallow dome-shaped top, which tapers to a thin peripheral edge of a small radius of curvature juxtaposed to the bottom and merging curvelinearily therewith. The substantially flat bottom surface can be of metal, epoxy or other suitable inert coating. Further, the bottom surface is provided with a catheter storage means, including rims or grooves. When implanted the casing is located with the dome-shaped top facing the skin, and the substantially flat surface supported by muscles or ribs.
The method widely used today for preventing generation of electric fields in regions where they could cause unwanted stimulation of muscles, by covering portions of a housing, which serves as an electrode, with an insulating material such as parylene, is an effective method. However, it is costly due to the step of the application of the parylene itself and the pre-treatment and after treatment steps.